1. Field of the Invention
The present invention relates to a semiconductor laser element including an ARROW (Antiresonant Reflecting Optical Waveguide) structure. In particular, the present invention relates to a semiconductor laser element including an ARROW structure and emitting laser light in the 980 nm band.
2. Description of the Related Art
A reliable high-power semiconductor laser element which emits a high-quality, diffraction-limited beam is required for use as a light source in exciting an optical fiber amplifier.
U.S. Pat. No. 5,606,570 discloses a semiconductor laser element having an ARROW structure as a semiconductor laser element which can emit a high-output-power, diffraction-limited laser beam in the 980 nm band. The disclosed semiconductor laser element includes an InGaAs active layer and an InGaAlP current confinement layer, and uses GaAs as a medium having a high refractive index. The ARROW structure is a structure for confining light in core regions. The disclosed ARROW structure includes a plurality of core regions having a low equivalent (effective) refractive index, first high-refractive-index regions which have a high equivalent refractive index and are arranged between the plurality of core regions and on the outer sides of the plurality of core regions, low-refractive-index regions which have an equivalent refractive index approximately identical to that of the plurality of core regions and are arranged on the outer sides of the outermost ones of the high-refractive-index regions, and second high-refractive-index regions which have a high equivalent refractive index and are arranged on the outer sides of the low-refractive-index regions. The first high-refractive-index regions behave as reflectors of light in the fundamental mode, and the low-refractive-index regions suppress leakage of light. Thus, the semiconductor laser element can be controlled so as to operate in the fundamental transverse mode.
In addition, it is reported that a preferable value of the width db1xe2x80x2 of each of the outermost ones of the first high-refractive-index regions is determined in accordance with the equation (1), a preferable value of the width db2xe2x80x2 of each of the first high-refractive-index regions arranged between the plurality of core regions is determined in accordance with the equation (2), and a preferable value of the width of each of the low-refractive-index regions is dcxe2x80x2/2, where dcxe2x80x2 is the width of each of the plurality of core regions. In the equations (1) and (2), xcex is the oscillation wavelength, ncxe2x80x2 is the equivalent refractive index of the plurality of core regions, and nbxe2x80x2 is the equivalent refractive index of the first high-refractive-index regions.                               d          b1          xe2x80x2                =                              3            ⁢            λ                                4            ⁢                                          {                                                      n                    b                    xe2x80x22                                    -                                      n                    c                    xe2x80x22                                    +                                                            (                                              λ                                                  2                          ⁢                                                      d                            c                            xe2x80x2                                                                                              )                                        2                                                  }                                            1                2                                                                        (        1        )                                          d          b2          xe2x80x2                =                  λ                      2            ⁢                                          {                                                      n                    b                    xe2x80x22                                    -                                      n                    c                    xe2x80x22                                    +                                                            (                                              λ                                                  2                          ⁢                                                      d                            c                            xe2x80x2                                                                                              )                                        2                                                  }                                            1                2                                                                        (        2        )            
In order to produce an ARROW structure, it is necessary to use a regrowth technique. However, in the semiconductor laser element disclosed in U.S. Pat. No. 5,606,570, GaAs and InGaP layers (or InAlP, GaAs, and InGaP layers) are exposed at the surface as a base of the regrowth. Therefore, P-As interdiffusion occurs at the exposed surface during a process of raising temperature for the regrowth, and thus the regrowth is likely to become defective. As a result, the above semiconductor laser element is not actually used. Further, since the difference in the bandgap between the optical waveguide layer and the active layer is small, the above semiconductor laser element has poor temperature characteristics.
An object of the present invention is to provide a semiconductor laser element which includes an ARROW structure and is reliable in a wide output power range from low to high output power levels.
(I) According to the present invention, there is provided a semiconductor laser element comprising: a GaAs substrate of a first conductive type; a lower cladding layer formed above the GaAs substrate and made of AlxGa1xe2x88x92xAs of the first conductive type, where 0.57xe2x89xa6xxe2x89xa60.8; a lower optical waveguide layer formed above the lower cladding layer and made of In0.49Ga0.51P which is undoped or the first conductive type; a compressive-strain quantum-well active layer formed above the lower optical waveguide layer and made of undoped Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1, where 0.49y1 less than x1xe2x89xa60.4 and 0xe2x89xa6y1xe2x89xa60.1; an upper optical waveguide layer formed above the compressive-strain quantum-well active layer and made of In0.49Ga0.51P which is undoped or a second conductive type; a first etching stop layer formed above the upper optical waveguide layer and made of GaAs of the second conductive type; a second etching stop layer made of Inx8Ga1xe2x88x92x8P of the second conductive type and formed above the first etching stop layer other than stripe areas of the first etching stop layer corresponding to at least one current injection region and low-refractive-index regions located on outer sides of the at least one current injection region and separated from the at least one current injection region or outermost ones of the at least one current injection region by a predetermined interval, where 0xe2x89xa6x8xe2x89xa61, and the stripe areas of the first etching stop layer extend in a direction of a laser resonator; a first current confinement layer made of GaAs of the first conductive type and formed above the second etching stop layer; a third etching stop layer made of Inx9Ga1xe2x88x92x9P of the second conductive type and formed over the first current confinement layer and the stripe areas of the first etching stop layer, where 0xe2x89xa6x9xe2x89xa61; a fourth etching stop layer made of GaAs of the second conductive type and formed above the third etching stop layer other than at least one stripe area of the third etching stop layer corresponding to the at least one current injection region; a second current confinement layer made of In0.5(Ga1xe2x88x92zAlz)0.5P of the first conductive type and formed above the fourth etching stop layer, where 0.1xe2x89xa6zxe2x89xa61; a first upper cladding layer of the second conductive type, formed above the second current confinement layer and the at least one stripe area of the third etching stop layer, and made of one of AlGaAs and In0.5(Ga1xe2x88x92zAlz)0.5P, where 0.1xe2x89xa6zxe2x89xa61; and a contact layer made of GaAs of the second conductive type and formed above the first upper cladding layer.
The first conductive type is different in the polarity of carriers from the second conductive type. That is, when the first conductive type is n type, and the second conductive type is p type.
In addition, the term xe2x80x9cundopedxe2x80x9d means that a material is not doped with any conductive impurity.
Preferably, the semiconductor laser element according to the present invention may also have one or any possible combination of the following additional features (i) to (iv).
(i) Each of the at least one current injection region may have a width equal to or greater than 3 micrometers.
(ii) The semiconductor laser element according to the present invention may further comprise a second upper cladding layer formed between the upper optical waveguide layer and the first etching stop layer, and made of a material having identical composition and an identical conductive type to the first upper cladding layer.
(iii) The first current confinement layer may include first and second sublayers made of GaAs of the first conductive type, and a quantum-well layer formed between the first and second sublayers and made of an InGaAs material which has a smaller bandgap than the bandgap of the compressive-strain quantum-well active layer.
(iv) The semiconductor laser element according to the present invention may further comprise a lower barrier layer formed between the lower optical waveguide layer and the compressive-strain quantum-well active layer, and an upper barrier layer formed between the upper optical waveguide layer and the compressive-strain quantum-well active layer, where each of the lower barrier layer and the upper barrier layer is made of undoped Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2, where 0xe2x89xa6x2xe2x89xa60.3 and 0xe2x89xa6y2xe2x89xa60.6.
(II) The advantages of the present invention are as follows.
(i) Since the semiconductor laser element according to the present invention has the aforementioned construction, in the direction perpendicular to the thickness direction and the light propagation direction in the active layer, first high-refractive-index regions which have a relatively high equivalent refractive index are realized between at least one core region (corresponding to the at least one current injection region each of which has a stripe shape) and on the outer sides of the at least one core region, low-refractive-index regions which have a relatively low equivalent refractive index are realized on the outer sides of the outermost ones of the first high-refractive-index regions, and second high-refractive-index regions which have a relatively high equivalent refractive index are realized on the outer sides of the low-refractive-index regions. That is, the aforementioned ARROW structure is realized.
Since the semiconductor laser element according to the present invention includes the ARROW structure, the semiconductor laser element according to the present invention can emit a single peak beam in a transverse mode which is more effectively controlled than that in semiconductor laser elements which do not include the ARROW structure, even when the stripe width is increased.
In order to effectively control the transverse mode oscillation in the semiconductor laser elements which do not include the ARROW structure, the stripe width is required to be reduced to 3 micrometers or smaller, i.e., the width of the active region is required to be reduced. Therefore, when the output power is increased, the optical density in the active layer increases, and thus facet degradation is likely to occur. Consequently, the semiconductor laser elements which do not include the ARROW structure cannot operate with high output power in an effectively controlled transverse mode.
On the other hand, since the semiconductor laser element according to the present invention includes the ARROW structure, light can be satisfactorily confined in a wide stripe (active) region in the semiconductor laser element according to the present invention, and therefore the semiconductor laser element according to the present invention can emit laser light in the fundamental transverse mode from the wide active region.
(ii) Since the double-layer etching stop layers constituted by the InGaP layer and the GaAs layer are used, the precision in etching can be improved, i.e., it is possible to precisely form the distribution of the equivalent refractive index which realizes the ARROW structure.
(iii) In particular, when the width of the active region is increased to 3 micrometers or greater, the optical density in the active layer can be reduced, and therefore the temperature rise due to non-radiative recombination in vicinities of end facets can be suppressed. Thus, the semiconductor laser element according to the present invention can emit a laser beam in the fundamental transverse mode with higher power than the semiconductor laser elements which do not include the ARROW structure.
(iv) Since it is possible to form an internal stripe structure as well as the ARROW structure, the contact area between the electrode and the contact layer can be increased, and therefore the contact resistance can be reduced. Thus, it is possible to suppress the drop of optical output power due to heat generation.
(v) Since, in the semiconductor laser element according to the present invention, the optical waveguide layers are made of In0.49Ga0.51P, and the compressive-strain quantum-well active layer is made of undoped Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 (0.49y1 less than x1xe2x89xa60.4, 0xe2x89xa6y1xe2x89xa60.1), it is possible to increase the difference in the bandgap between the compressive-strain quantum-well active layer and the optical waveguide layers. Therefore, it is possible to prevent the carrier leakage, lower the threshold current, and improve the temperature characteristics.
(vi) In the case where a second upper cladding layer made of a material having identical composition and an identical conductive type to the first upper cladding layer is formed between the upper optical waveguide layer and the first etching stop layer, it is also possible to realize the distribution of the equivalent refractive index in the lateral direction perpendicular to the stripes which is necessary for the formation of the ARROW structure.
(vii) In the case where the first current confinement layer includes first and second sublayers made of GaAs of the first conductive type, and a quantum-well layer formed between the first and second sublayers and made of an InGaAs material which has a smaller bandgap than the bandgap of the compressive-strain quantum-well active layer, it is possible to increase the gain of oscillation in the fundamental transverse mode since the InGaAs quantum-well layer absorbs light.
(viii) In the case where barrier layers made of undoped Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 (0xe2x89xa6x2 less than 0.3, 0xe2x89xa6y2xe2x89xa60.6) are formed between the compressive-strain quantum-well active layer and the upper and lower optical waveguide layers, it is possible to achieve further higher performance.